Some 20 robotic vehicles will be steering themselves across the Mojave desert this weekend.

The robot racers are taking part in a Grand Challenge organised by the US Defence Advanced Research Projects Agency to drive work on robot vehicles.

The autonomous racers must make their own way across a course expected to be more than 200km in length.

The machine that finishes the course in the best time will win its creators a prize of $1m.

Starting line

The vehicles taking part range include a converted Humvee, a modified Honda Acura and a robot motorbike.

Some of the racers are custom built

The 21 teams behind the robot vehicles include enthusiastic amateurs, engineers from big hi-tech firms and several student groups.

Initially 25 teams entered the competition but four dropped out even before a qualifying competition took place.

Only one robot racer, called Sandstorm, managed to complete the short 1.5km qualifying course, which was designed to find out which vehicles were fit to take on the longer course.

Vehicles of eight other teams managed to complete a small part of the course that featured bricks, metal rods and other obstacles to challenge the ability of the robot cars to navigate difficult terrain.

Most of the robots use a combination of sensors to work out what is ahead of them and combine it with GPS information to plot their route over the larger course.

Darpa has now changed the rules of the qualifying event so all the teams that made it to the qualifying event will be on the start line on 13 March.

Final route

Shortly before the start of the official race on Saturday competing teams will be given a CD laying out the full course their robot racer must cover.

The Mojave desert is a challenge for any vehicle

Several possible routes are being plotted by Darpa over the course that stretches across rugged desert terrain between Barstow, California and Primm, Nevada.

Vehicles must complete the course in less than 10 hours. The final distance to be travelled could vary from 241 to 338km.

Each robot traveller must complete the course without aid from its creators. It will be accompanied by a Darpa vehicle, driven by humans, who will hit a kill switch if a racer runs amok.

Darpa is maintaining a live webpage that show spectators where each vehicle is on the course on a map.

If no robot vehicle manages to complete the course in the time limit, the race looks likely to be re-run in 2006.

The race is designed to help Darpa develop a series of autonomous vehicles that can re-supply US troops on the battlefield.

Darpa wants to have a third of all combat vehicles autonomous by 2015.

The distance to be travelled by the robot racers is a fraction of that travelled by an autonomous flying robot.

In April 2001 the Global Hawk Unmanned Aerial Vehicle flew 13,840km (8,600 miles) in 22 hours across the Pacific from California to Australia.